This invention relates to an environmentally acceptable process for the extraction and recovery of gold and platinum group metals from ores containing low concentrations of these precious metals.
Traditionally the recovery of gold and the platinum group metals from ores containing these metals has been achieved by extraction with cyanides, by mercury amalgamation, flotation, and electrolytic separation and electrostatic separation. In the electrostatic separation method, the metals in the dry ore are electrostatically charged, usually by passing a high velocity arc through the ore and then the precious metals are recovered on an oppositely charged collector. In electrolytic separation, the metals in an ore are dissolved and then electroplated on an oppositely charged electrode. Other methods of separation that have been used include gravimetric differential density separation, chlorination, ion exchange, and other procedures both mechanical and chemical.
All of the aforementioned processes are costly, time consuming, inefficient, and environmentally polluting as they recover only a small fraction of the precious metals in the ore, or require the use of toxic chemicals which can be injurious and damaging to plant and animal life when discharged to the environment.
Accordingly, there exists a need for a method, which achieves greater recoveries of precious metals than the currently available practiced methods. There is further a need for a simple, low-cost, non-polluting, and efficient method for the extraction of gold and the platinum group metals from ores without the use of toxic chemicals such as mercury, cyanides and strong acid leaches that persist for long periods of time. A less costly process in plant cost and labor than conventionally used mechanical procedures is highly desirable. Additionally a need exists for a process where the recovery of precious metals occurs from low concentration sources, such as mine xe2x80x9ctailingsxe2x80x9d. Another highly desirable feature is the remediation of the aforementioned sources such as tailings such that they are rendered useful, thus simplifying disposal of the residual ores or reprocessed mine tailings.
It is an objective of this invention to provide a method, which achieves greater recoveries of gold, and platinum group metals than currently practiced methods from low concentration sources.
It is an added objective of this invention to provide a composition of innocuous chemicals at the completion of this process, which are useful as an extractant for precious metals in sources with low concentrations.
It is a further objective of this invention to provide a simple, low cost, non-polluting, and efficient method for the extraction of gold, and the platinum metals from low concentration sources without the use of toxic chemicals such as mercury, cyanides and strong acid leaches that will be discharged into the environment.
It is likewise an objective of this invention to provide a precious metal recovery method which poses no threat to the environment or ground water and provides a process which remediates such low concentration sources as mine tailings such that they can be used in other processes such as pozzolanic materials for use in concrete.
This invention comprises a method for the extraction of gold and the platinum group metals (ruthenium, rhodium, palladium, osmium, iridium, and platinum) from, but is not limited, to low concentration ores, recovered catalysts and other sources by the use of inexpensive and small quantities of common chemicals, non-toxic to the environment or ground water. The invention is best described with reference to FIGS. 1 through 3, which are process flow diagrams of the precious metal recovery method of this instant invention.
The source material is comminuted prior to entry into the initial extraction tank. Some ores required thermal shocking to allow more of the ore to be extracted by the leaching solution. This solution comprises a mixture of hydrochloric acid (HCl) and sodium hypochlorite (NaOCl) which under acidic conditions is converted to chlorine (Cl2). All steps involving the use of HCl and NaOCl are carried out in enclosed systems under negative pressure where any fumes are scrubbed through a spray mist of sodium hydroxide, sodium carbonate, or similar salts or mixtures thereof. The required Cl2 is created within the extraction vessel thereby eliminating the need for the handling of Cl2. The Cl2 only exists within the enclosed vessel for a short period of time until the desired metals are oxidized to the chloride salts.
After extraction the liquid can be decanted and the xe2x80x98bottoms, the residual material that contains the extracted ore, are sent to a counter current extractor prior to drying for use as soil amendments or concrete additives. Any residual salt in the extracted ore is washed in the counter current extractor.
Neutralization of the liquid generates base metal precipitates as hydroxides. The precious metals remain in the liquid. The base metal precipitates are filtered. Residual base metals in the solution with the desired metals are further extracted through a strong acid cation resin prior to treatment by a reducing agent. Treatment of the filtered leach solution with a reducing agent precipitates the precious metals. The precious metals at this point are a mixture of the desired metals. They can be commercially utilized by conventional techniques such as smelting for further purification. In the desired process, however, they are redissolved in HCl with sufficient oxidizing agent (NaOCl can be used again) to convert them back to the chloride salts. The precious metals existing as chloride salts are separated by chromatography. After separation the precious metal salts are reduced to the pure metal by hydrogen reduction.